We had a software package that had a braindead licensing scheme. To generate the license, it uses the MAC address of the network interface card of the machine you are running it on. OK, that's a way of identifying a unique machine. But here's the kicker. It just assumes that your ethernet device is /dev/eth0.
For a while now, NICs that are embedded on the motherboard are identified by udev as em1, em2, etc. This is part of an attempt to make interface naming more predictable and meaningful.